A light emitting diode (LED) indicates a kind of semiconductor device that may implement light having various colors by configuring a light source through the PN diode formation of a compound semiconductor. Such a light emission element has advantages in that it has a long life, may be decreased in size and weight and driven at a low voltage. Also, such an LED is resistant to a shock and vibration, does not need a preheating time and complex operation, is mounted on a substrate or lead frame in various shapes, and then may be packaged. Thus, it is possible to modularize the LED for many uses and apply it to a backlight unit or various lighting devices.
A plurality of LEDs may be used in order to provide single independent lighting, in which case the LEDs may be connected in series or in parallel with each other. In this case, in order to always keep all of the LEDs being turned on, it is possible to convert commercial AC power supply into DC power and apply the DC power to the LEDs.
The method above needs a separate DC rectifier when the DC power is supplied, but other methods may apply the AC power supply directly to the LEDs without the DC rectifier. In this case, the LEDs may be connected in series with each other and the ON/OFF state of each of the LEDs may vary according to the size of a variable input voltage. Thus, there are limitations in that flicker occurs due to the repetition of ON/OFF state, the availability of each LED decreases and thus light output efficiency decreases.
Although the lighting device including the LEDs is driven with the AC power supply, it may be helpful to use the AC power supply without using a DC power supply device (1) if it is possible to remove or mitigate flicker and (2) if it is possible to prevent a decrease in power factor according to an AC power supply operation.
The peak voltage of the commercial AC power supply may depend on the region. In this case, when a single lighting device using LEDs is applied to AC power supply having different sizes, the brightness of the lighting device may vary and power efficiency may also vary. Thus, there is a need for LED lighting for AC power supply that may represent uniform light output and efficiency even when AC power supply having different sizes is applied.